1. Field of the Invention
This invention relates to a filter for collecting [dry soot, fine particles and SOF (Soluble Organic Fraction), etc,]contained in an exhaust gas of a Diesel engine. Hereinafter, "particulate matter"is referred to as "PM" for abbreviation.
2. Description of the Prior Art
Exhaust gases of automobiles are one of the major causes for air pollution, and technology for removing noxious components contained in the exhaust gases is extremely important. Particularly, in Diesel engine vehicles, removal of PM containing carbon fine particles as a principal component is very important. To remove these noxious components, engine improving attempts have been made by applying EGR (Exhaust Gas Recirculation) and improving a fuel injection system and the shape of a combustion chamber. Nonetheless, a drastic measure has not yet been established. Therefore, a method which disposes an exhaust trap in an exhaust passage, collects PM by the trap and removes them by after-treatment has been proposed (Japanese Patent Laid-Open No. 51235/11983). This after-treatment method has been believed most practical to this date, and further studies have been continued.
The particulate trap for collecting the PM must satisfy the following performance as required by the conditions of use of the trap.
First, the particulate trap must have PM collection efficiency to satisfy a required purity of the exhaust gas. The PM emission quantity Varies with an exhaust quantity and a load of the Diesel engine, but it is believed that the trap must collect at least 60% of the mean PM emission quantity (at least 85% of dry soot collection efficiency) from the Diesel engine.
Second, a pressure loss to the exhaust gas must be small. As more PM is collected, the pressure loss at the time when the engine exhaust gas passes through the trap becomes greater, so that a back pressure acts on the engine and exerts adverse influences on the engine. It is generally believed that the pressure loss after collection must be limited to not higher than 3 kPa. To satisfy this requirement, the filter element must be periodically cleaned to remove the PM whenever a predetermined quantity of the PM is collected, so as to regenerate the filter element and to return it to the initial pressure loss state. When the ratio of the rise of the pressure loss to the PM collection quantity is great, this regeneration/removal operation becomes more frequent and is not practical. Accordingly, the particulate trap must not only have a small initial pressure loss but must also prevent easy rise of the pressure loss even after the PM in the exhaust gas is collected.
Third, the trap must have durability sufficient to withstand the regeneration/removal operation described above which is repeatedly carried out. A burning-removal method which burns and removes the PM by electric heating or burner heating is believed the most effective regeneration method, but in either of the systems, heating is made to a temperature (about 600.degree. C.) or above at which the PM is ignited. Regeneration is executed before the drop of engine performance or a trouble of the operation due to the back pressure occurs, and the PM is burnt and disposed. Thereafter, the PM is again collected, and regeneration of the trap and collection are again carried out, so that the pressure loss can be always kept below a predetermined level. For this reason, heat-resistant materials capable of withstanding repeated regeneration processing must be selected as the filter element materials, and corrosion resistance to the atmospheric gas contained in the exhaust gas is also necessary.
Conventionally, a wall flow system honeycomb-like porous body of cordierite ceramics has been believed to be most practical candidate for the filter element material that satisfies the requirements described above. According to this system, however, the PM is likely to locally gather. Further, because the cordierite ceramics have a low thermal conductivity, heat spots are likely to be formed at the time of regeneration, and the filter is molten and lost or cracks occur due to a thermal stress. For these reasons, reliability cannot be secured.
To solve the problems described above, a honey-comb-like porous body using SiC ceramics is used for the filter element. This filter element can prevent melt-loss and cracks at the time of regeneration due to a high thermal conductivity of SiC, but has not yet been put into practical application because a greater calorie is necessary at the time of regeneration (Japanese Patent Laid-Open No. 23512/1993).
Diesel particulate filters and their system which can be regenerated without the occurrence of melt-loss and cracks by using a foamed metal have been investigated. However, the foamed metal has a mean skeletal thickness of at least 80 .mu.m.phi. and a mean pore diameter of at least 200) .mu.m.phi.. Therefore, even when the foamed metal is rolled to change the pore shape, a dry soot collection efficiency of at least 75% cannot be attained (Japanese Patent Laid-Open No. 86313/1992), and another problem remains unsolved in that collection efficiency greatly fluctuate depending on the engine condition.